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64
C
HAPTER
4: V
IRTUAL
LAN
S
(VLAN
S
)
■
VLANs ease the change and movement of devices.
With traditional networks, network administrators spend much of
their time dealing with moves and changes. If users move to a
different subnetwork, the addresses of each endstation must be
updated manually.
For example, with a VLAN, if an endstation in VLAN
Marketing
is
moved to a port in another part of the network, and retains its original
subnet membership; you must only specify that the new port is in
VLAN
Marketing
.
IGMP Overview
IGMP is a protocol used by an IP host to register its IP multicast group
membership with a router. The messaging protocol can also be
“snooped” by a layer 2 switch, to provide for intelligent forwarding of
multicast data streams within a VLAN. Periodically, the router queries the
multicast group to see if the group is still in use. If the group is still active,
a single IP host responds to the query, and group registration is
maintained.
IGMP Snooping
IGMP snooping is a layer 2 function of the switch. The feature reduces
the flooding of IP multicast traffic, optimizes the usage of network
bandwidth, and prevents multicast traffic from being flooded to parts of
the network that do not need it. The switch does not reduce any IP
multicast traffic in the local multicast domain (224.0.0.x). An optional
optimization for IGMP snooping is the strict recognition of multicast
routers only if the remote devices have joined the DVMRP (224.0.0.4) or
PIM (244.0.0.13) multicast groups.
IGMP snooping is enabled by default on the switch. If IGMP snooping is
disabled, all IGMP and IP multicast traffic floods within a given VLAN. This
is standard 802.1d bridge behavior . IGMP snooping expects to see
periodic IGMP reports from interested hosts on each port. Without an
IGMP querier, the switch may stop forwarding IP multicast packets to all
ports.
To support IGMP snooping in environments that do not have an IGMP
querier, the switch can function as an IGMP querier, per the rules of
standard IGMP Version 2.0. If IGMP snooping is enabled, the switch
periodically queries for multicast group memberships. However, if either
IGMP snooping is disabled or IGMP functionality is disabled, the switch
does not generate IGMP query messages. IGMP should be enabled when